Heyo. I've got some not-so-great news...

I tried activating the key, but only got the usual duplicate activation message: "The product code you've entered has already been activated by a different Steam account. This code cannot be used again. Please contact the retailer or online seller where the code was purchased for assistance"

If you're sure you didn't use the key already, you're probably best off contacting HumbleBundle support. They are one of the few places that actually can revoke the original keys they've issued most of the time, so they're usually very helpful and quick to replace an invalid key for a working one.